This invention relates to pressure reducing elements and, more particularly, to variable pressure reducing elements adapted for use in sampling systems for high pressure fluids.
In the operation of same high pressure fluid systems, such as steam generating systems for power plants, samples of the fluid are periodically analyzed for various properties and/or composition. The fluid being sampled often is at several thousand pounds pressure and this pressure must be reduced to a relatively low pressure, for example, 25 psi, prior to introduction into the analyzer. This pressure reduction desirably should be effected under laminar flow conditions in order to minimize disassociation of components in the fluid sample which can be caused by discontinuous pressure drops.
Valves including an orifice seat, such as needle valves, generally are not acceptable for this purpose because of a tendency to create turbulent flow and, more importantly, because the high velocity flow through the constricted area between the seat and the valve member causes rapid seat wear, even though it is constructed from a hard, wear resistant material.
Devices including a capillary-like tube several feet long and arranged to provide a torturous flow path have been used for this purpose. However, such devices can not be conveniently cleaned in the event the tube becomes plugged with foreign particles and the pressure reduction can not be adjusted.
Pressure reducing elements including a stepped cylindrical valve rod disposed in a small tube and axially movable therein to adjust the flow area between the tube bore and the valve rod have been used for this purpose. In the event such an element becomes plugged with foreign solid particles, such as rust, it usually is necessary to shut off the sample flow and remove the valve rod for cleaning.
This problem can be alleviated by using a double rod and tube arrangement such as that disclosed in Sentry Equipment Corp. Bulletin 1.10.1 RO 5-79, copy of which is enclosed. In such an arrangement, the high pressure fluid enters one tube, flows past the valve rod therein, makes a U-turn, flows past the valve rod in the other tube, and finally exits from the other tube. The valve rods are connected to a spindle which can be rotated by an external knob to retract the rods, without shutting off the sample flow, to a position where the system pressure can "blow" or flush the foreign particles out the exit tube. The external knob is also used for adjusting the pressure reduction provided by the element. Consequently, each time the rods are retracted to flush out plugging foreign particles, it is necessary to re-adjust them to the desired pressure reducing position and this task can be quite time consuming. Also, such a device and the other type devices discussed above have a relatively large internal flow volume which can be disadvantageous when used with a fluid containing radioactive materials, such as steam from a nuclear power plant, because the amount of shielding required to meet saftey standards correspondingly increases with increases in internal flow volume.
Pressure reducing elements for sampling systems desirably should be capable of providing a minimum sample flow to the analyzer when the system being sampled is at full operating pressure and at a relatively low pressure such as after shutdown.
U.S. Pat. No. 4,099,703 discloses a self-cleaning needle valve including means for retracting the valve stem without affecting the flow adjustment position. However, a spring biasing the valve stem to an adjusted position is located inside the valve body where it can be exposed to the fluid being sampled. Also, only a small portion of the valve stem normally exposed to the fluid sample can be retracted from the valve body. Consequently, it cannot be retracted for convenient inspection and cleaning.
Representative prior valve arrangements including a stepped bore or a stepped valve stem are disclosed in U.S. Pat. Nos. 2,725,212 (Jensen), 2,790,463 (Delano et al.) and 3,365,166 (Smith).